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Nucleophilic Substitution Answers

Qu 1:

2-methyl-2-propanol or t-butanol (3o), most reactive 
2-butanol (2o) 
2-methyl-1-propanol (1o) 
1-butanol (1o), least reactive 

Since the reaction of an alcohol with HBr favours an SN1 type reaction, the systems generating the more stable carbocation will react fastest.
Qu 2:

(a) Since the -OH group is such a poor leaving group, it can't be displaced directly.  In the reaction with HBr the -OH is protonated first and the leaving group is a neutral water molecule which is much better. In the reaction with NaBr, there is no acid catalyst in order to activate the leaving group and therefore no reaction is observed.


(b) In the presence of sulfuric acid, H2SO4, the -OH becomes protonated and can now leave to be replaced by the bromide ion providing the alkyl bromide.
organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary